In the processing of plastics, metals, alloys etc., internal stresses occur during solidification. They are largely retained in the finished moulding or casting, and are responsible for quality deterioration. These stresses are concentrated at certain places in a cast, injection-moulded or pressed part, and are caused mainly by unequal shrinkages during solidification.
Hitherto it has not been possible to detect such stresses non-destructively, neither during the production process nor after completion of the finished product.
At the present time, internal material stresses can be measured only with a complex drilling technique, and only to a limited extent. The suspected critical stress point is drilled out in stages. During this operation, the surroundings are tested for stress changes with strain gauges applied. It is thus possible today to assess internal stresses resulting from the production process only by destroying the casting or moulding. This is a lengthy and costly procedure.
The device according to the invention uses new approaches to measure, for the first time, the shrinkage or expansion process during the solidification of the casting or moulding in its mould. The solidification process (i.e. the transition from liquid or viscous to the solid state) is accompanied by crystal transformations and hence volume changes is known. In metal powder injection moulding (MIM), the shrinkage factor may amount to as much as 20%.
That the accompanying shrinkage or expansion (for example when water freezes into ice) can cause considerable stresses in the casting or moulding is also well known. Knowledge of these stress processes and means of monitoring them thus open up new possibilities for quality control.
New and in accordance with the invention therefore is the utilization of this exactly limited phenomenon in time during the production process to measure the changes in the stresses at predetermined points on the inner wall of the injection or casting mould. A measuring device has a measuring surface fitted into the injection or casting mould flush with the workpiece surface, so that this can move freely in all three orthogonal directions, i.e. it is separated from the mould wall by a minimal air gap. This air gap is so small that the material cannot pass through it in liquid or viscous state, even though it is under high pressure. The gap width depends on the viscosity of the fluid material, and can be determined by empirical tests. The measuring surface of the device is preferably the front face of a sensor screwed into the mould, which measures shear and compressive forces and is able to convert these into voltage changes with the appropriate electronics. However other measuring surfaces are conceivable and in accordance with the invention.
In the figures that follow, the invention is represented as a typical application in plastics injection moulding technology. However the invention is also applicable in metal pressure die-casting and injection moulding, and in a number of other processes where shrinkage phenomena have to be detected.
The process and device will be explained with reference to the figures.